Seismic exploration involves surveying subterranean geological media for hydrocarbon deposits. A survey typically involves deploying seismic sources and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological medium creating pressure changes and vibrations. Variations in physical properties of the geological medium give rise to changes in certain properties of the seismic waves, such as their direction of propagation and other properties.
Portions of the seismic waves reach the seismic sensors. Some seismic sensors are sensitive to pressure changes (e.g., hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy one type of sensor or both. In response to the detected seismic waves, the sensors generate corresponding electrical signals, known as traces, and record them in storage media as seismic data. Seismic data will include a plurality of “shots” (individual instances of the seismic source being activated), each of which are associated with a plurality of traces recorded at the plurality of sensors.
In some cases, it is desirable to analyze the recorded seismic amplitudes. This may be done in many ways. One step in conventional processing of seismic reflection data involves adding multiple seismic traces that share a common mid-point, but have different source-receiver offsets. This is commonly called “stacking”. Stacking generally improves the signal to noise ratio, but can result in ambiguity surrounding the cause of the seismic amplitudes. For example, a high seismic amplitude could indicate either the presence of fluids or the presence of a particular lithology.
One conventional technique that can provide an improved method of delineating between lithology and fluids is employment of amplitude versus offset (AVO) or angle (AVA) for a representative offset/angle gather. Those of skill in the art would be aware that amplitude versus angle (AVA) is often used interchangeably with amplitude versus offset (AVO).
During processing, this type of AVA data may not be stacked thereby to preserve information that can be used to distinguish indicators of fluids from indicators of lithology. For example, considering a seismic trace, in one scenario, a hydrocarbon-bearing sand may generally have an increasingly negative seismic amplitude at further source-receiver offsets compared to a water-bearing sand which may be indicated by a decrease in positive seismic amplitude at further source-receiver offsets.
The production of hydrocarbons causes changes in the elastic parameters of the earth. These changes may occur due to water displacing oil (or vice versa), water displacing gas (or vice versa), or gas displacing oil (or vice versa), within the reservoir interval. In other cases, the changes in the elastic parameters may occur due to enhanced hydrocarbon recovery operations, CO2 injection, or clathrate dissociation from solid to gas. Time-lapse (4D) seismic data is acquired to compare seismic data at different times via two or more seismic surveys, a seismic survey at time one (T1) and another seismic survey from time two (T2), conducted months or years apart. The differences in the seismic responses for T1 and T2 are at least partially due to fluid movement and/or pressure changes due to production or injection of water or gas. Conventionally, these differences in seismic response are qualitatively interpreted relative to modeled response behaviors due to fluid and pressure changes. Typically, the seismic survey from T1 is referred to as the baseline survey, and the seismic survey from T2 is referred to as the monitor survey. However, in the case for more than one monitor survey we could be analyzing two monitor surveys, where the seismic survey from T1 is an early monitor survey and the seismic survey from T2 is another monitor survey recorded at some time T2 where T2 is months or years after T1.
The above methods may however often be biased and may not truly represent the geologic features. In addition, conventional methods may fail where seismic data quality is low, such as where random and/or coherent noise is prevalent, or where seismic gathers are not flat. The ability to define the location of rock and fluid property changes in the subsurface is crucial to our ability to make the most appropriate choices for purchasing materials, operating safely, and successfully completing projects. Project cost is dependent upon accurate prediction of the position of physical boundaries and fluid content within the Earth. Decisions include, but are not limited to, budgetary planning, obtaining mineral and lease rights, signing well commitments, permitting rig locations, designing well paths and drilling strategy, preventing subsurface integrity issues by planning proper casing and cementation strategies, and selecting and purchasing appropriate completion and production equipment.
There exists a need for seismic processing methods capable of producing improved time-lapse AVA information that may be used for analysis of geologic features of interest.